Time Measurement Threatens Privacy-Friendly RFID Authentication - PowerPoint PPT Presentation

Time Measurement Threatens Privacy-Friendly RFID Authentication Protocols Gildas Avoine 1 , Iwen Coisel 2 and Tania Martin 1 1: Information Security Group - Universit´ e Catholique de Louvain 2: Crypto Group - Universit´ e Catholique de Louvain RFIDSec 2010 UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 1 Microelectronics Laboratory

The Privacy of an RFID Authentication Scheme ◮ Interest relative to the application ◮ not really necessary in inventory management ◮ essential in passport context to protect user’s identity and also to prevent anybody to trace him ◮ Lots of sensitive applications ◮ medical supplies ◮ transport cards ◮ luxury items ◮ ... ⇒ Real necessity of a privacy analysis We here focus on traceability UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 2 Microelectronics Laboratory

Privacy vs Time Measurement Several privacy models exist [A05,JW07,LBM07,V07,CCG10] ◮ Juels and Weis : possible to know the result of a protocol ◮ Vaudenay : tags are not necessary in the adversary’s field How long it takes to a reader to identify a tag ? None of them It’s not (only) an implementation issue UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 3 Microelectronics Laboratory

Privacy vs Time Measurement Several privacy models exist [A05,JW07,LBM07,V07,CCG10] ◮ Juels and Weis : possible to know the result of a protocol ◮ Vaudenay : tags are not necessary in the adversary’s field How long it takes to a reader to identify a tag ? None of them It’s not (only) an implementation issue Contributions : ◮ Point out this threatens ◮ Formalize it ◮ Attacks some protocols ◮ Present some countermeasures UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 3 Microelectronics Laboratory

Outline 1 � Modelling Privacy 2 � Time-Attack on Some Existing Schemes 3 � Countermeasures 4 � Conclusion UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 4 Microelectronics Laboratory

Outline 1 � Modelling Privacy 2 � Time-Attack on Some Existing Schemes 3 � Countermeasures 4 � Conclusion UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 5 Microelectronics Laboratory

Vaudenay’s Model [Vau07] List of oracles given to an adversary A ◮ CreateTag : adds a new legitimate tag. ◮ DrawTag : tag enters in the adversary’s field ◮ Free : tags goes out of the adversary’s field ◮ Execute : returns transcripts. ◮ Launch ◮ SendTag ◮ SendReader ◮ Result ◮ Corrupt : returns tag’s key set. UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 6 Microelectronics Laboratory

Vaudenay’s Model [Vau07] Considering the Corrupt oracle, 3 adversary’s ability : ◮ WEAK : no Corrupt allowed ◮ FORWARD : Corrupt “stops” the system ◮ STRONG : Corrupt has no effect Considering the Result oracle, 2 adversary’s ability : ◮ NARROW : no Result allowed Adversary classes ordered by power P STRONG ⇒ FORWARD ⇒ WEAK ⇓ ⇓ ⇓ N-STRONG ⇒ N-FORWARD ⇒ N-WEAK UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 7 Microelectronics Laboratory

Vaudenay’s Model [Vau07] Experiment of A 1. A interacts with the whole system 2. A submits an hypothesis 3. A obtains Tab and returns 0/1 The protocol is said P -private if A sim has the same success probability as A : | Pr [ A → 1] − Pr [ A sim → 1] | < ǫ ( k ) STRONG ⇒ FORWARD ⇒ WEAK ⇓ ⇓ ⇓ N-STRONG ⇒ N-FORWARD ⇒ N-WEAK UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 8 Microelectronics Laboratory

Time-Privacy To capture the time notion in an authentication protocol ◮ Timer : outputs the time δ taken by the reader for its overall computations during a given protocol instance Possible to define the TIMEFUL-Privacy ◮ Adds a new ability ⇒ more powerful ◮ At each level X ∈ { STRONG, FORWARD, WEAK } : TIMEFUL- X ⇒ X ⇓ ⇓ TIMEFUL-NARROW- X ⇒ NARROW- X UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 9 Microelectronics Laboratory

Outline 1 � Modelling Privacy 2 � Time-Attack on Some Existing Schemes 3 � Countermeasures 4 � Conclusion UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 10 Microelectronics Laboratory

Context of the Study Several key infrastructures possible secret-key public-key master X Yes particular Yes Yes Considering Vaudenay’s generic scheme [Vau07] ◮ Authentication : encryption of ID|| K || a ◮ Verification : decryption of the message + authenticity of K ⇒ constant-time authentication Particular secret-key infrastructure ◮ Each tag owns a particular secret-key ◮ The reader does not know which key to use ⇒ SearchID procedure UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 11 Microelectronics Laboratory

WSRE Protocol Protocol proposed by Weis, Sarma, Rivest and Engels [WSRE03] ◮ Each tag owns a secret key sk ID ; ◮ f is a pseudo-random function ; SearchID procedure : brute-force search UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 12 Microelectronics Laboratory

WSRE Protocol Protocol proposed by Weis, Sarma, Rivest and Engels [WSRE03] ◮ Each tag owns a secret key sk ID ; ◮ f is a pseudo-random function ; SearchID procedure : brute-force search ◮ Best case : 1 computation ◮ Average : n / 2 computations ◮ Worst case : n computations UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 12 Microelectronics Laboratory

WSRE Protocol A time-attack on WSRE ◮ A creates 2 legitimate tags and affects them : t 1 and t 2 ◮ A calls Execute (t 1 ) and Execute (t 2 ) : ( π 1 , tr 1 ), ( π 2 , tr 2 ) ◮ A calls Timer ( π 1 ) and Timer ( π 2 ) : δ 1 and δ 2 ◮ A frees both tags, and reaffects only one of them : t 3 ◮ A calls Execute (t 3 ) : ( π 3 , tr 3 ) ◮ A calls Timer ( π 3 ) : δ 3 ◮ If δ 3 = δ 1 , then t 1 = t 3 , else t 2 = t 3 ⇒ Pr [ A → 1] = 1 UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 13 Microelectronics Laboratory

WSRE Protocol A time-attack on WSRE ◮ A creates 2 legitimate tags and affects them : t 1 and t 2 ◮ A calls Execute (t 1 ) and Execute (t 2 ) : ( π 1 , tr 1 ), ( π 2 , tr 2 ) ◮ A calls Timer ( π 1 ) and Timer ( π 2 ) : δ 1 and δ 2 ◮ A frees both tags, and reaffects only one of them : t 3 ◮ A calls Execute (t 3 ) : ( π 3 , tr 3 ) ◮ A calls Timer ( π 3 ) : δ 3 ◮ If δ 3 = δ 1 , then t 1 = t 3 , else t 2 = t 3 ⇒ Pr [ A → 1] = 1 For the simulation, the output of Timer ( π 3 ) is guessed ⇒ Pr [ A Sim → 1] = 1 / 2 WSRE is NOT TIMEFUL-WEAK-private. UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 13 Microelectronics Laboratory

Several Attacks Ohkubo, Suzuki and Kinoshita [OSK03] ◮ NARROW-FORWARD private ◮ Not TIMEFUL-WEAK private ◮ Desynchronisation helps to distinguish two tags Undesynchronizable schemes [D05, LBM07, CC08, ...] ◮ Only one possible desynchronization ◮ WEAK private ◮ Not TIMEFUL-WEAK private UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 14 Microelectronics Laboratory

Outline 1 � Modelling Privacy 2 � Time-Attack on Some Existing Schemes 3 � Countermeasures 4 � Conclusion UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 15 Microelectronics Laboratory

Presentation Major concern = SearchID procedure Example for WSRE ◮ Always waiting until the worst case ( n computations) ◮ “Always” applicable ◮ Not efficient ◮ Random SearchID instead of a linear one ◮ More efficient : n / 2 computations in average for each tag Countermeasures ◮ Not possible to link a time length to a tag ◮ Optimally : time length independent of n UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 16 Microelectronics Laboratory

Undesynchronizable Schemes Tags can be desynchronized once ⇒ 2 possible keys per legitimate tag ◮ Worst case : 2 n computations (instead of n ) ◮ Random Search ◮ Synchronized tag : n / 2 computations ◮ Desynchronized tag : 3 n / 2 computations ⇒ A can distinguish 2 tags ◮ New Random Search ◮ Random among the whole set of keys (current and old/next ones) ◮ Average time for all tags : n computations UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 17 Microelectronics Laboratory

Precomputation Solution No random values in OSK ⇒ Precomputation of “all” answers possible : n . m answers ◮ Balanced Binary Search ◮ SearchID efficient : O (log n ) ◮ really dynamic : tags can be added infinitely ◮ Rainbow Table [AO05,ADO05] ◮ Database size reduced ◮ Efficiency of SearchID depends on the time-memory trade-off ◮ But not dynamic ◮ But requires database update (instead of tag update) UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 18 Microelectronics Laboratory

Outline 1 � Modelling Privacy 2 � Time-Attack on Some Existing Schemes 3 � Countermeasures 4 � Conclusion UCL Crypto Group Avoine - Coisel - Martin Time Measurement - RFIDSec 2010 19 Microelectronics Laboratory